The present invention relates to a method of cementing wells, such as oil wells, gas wells or water wells penetrating underground formations. To exclude fluids from the annular space around a casing string or other pipe placed in the well, a cement slurry is placed in the annular space and the cement, after setting thereof, will seal the passage through the annulus and bond the casing string to the wall of the well. The cement slurry may be pumped directly into the annulus, or may first be passed downwards through the casing string (or through a special cementing tube suspended in the string) and subsequently upwards through the annular space around the casing string.
During the cementing operations, care should be taken that the pressure at each level of the annulus is less than the fracturing pressure at the relevant depth level, since the formation will otherwise be fractured and the cement slurry will pass into the formation rather than filling up the annulus around the casing. To obviate this problem, which is in particular present when cementing wells that penetrate underground formations located below a body of water, such as a sea or ocean, so-called lightweight cements have been developed. The cement slurries of the lightweight type have a density that is considerably lower than the density of the normal cement slurries, such as in the range of 900-1900 kg/m.sup.3, whereas the density of a normal cement slurry is about 1920 kg/m.sup.3.
The density decrease of cements is often obtained by adding granular material of low density to a pumpable cement slurry. Such material may be bentonite, coal, asphalt, fired clay and void-containing particles that cannot be wetted by water (such as hollow sealed ceramic spheres). Care should then be taken that the compressive strength of the cement after setting is not decreased to an undesirable extent.
Another attractive manner of reducing the density of a cement slurry is by adding gas (such as air and nitrogen) to the cement slurry in a manner such that the gas forms a fine dispersion in the slurry, which dispersion may be stabilized by means of suitable foaming agents.
Various procedures have been proposed for forming porous or other types of lightweight cements for the above or other uses. For example, a lightweight bitumen or aluminum silicate-containing "Libit" cement for sealing along weak formations is described in the V. P. Quist/J. J. M. Zuiderwijk U.S. Pat. No. 3,887,385. A lightweight low water-loss cement containing a low density calcined shale cement, attapulgite and water, which is said to provide improved sealing along gas-producing sands, is described in U.S. Pat. No. 4,120,360. U.S. Pat. No. 3,926,257 suggests sealing along such gas sands with a cement slurry which contains a surfactant for converting any gas which diffuses into the slurry into an immobile foam. Numerous U.S. Pat. Nos. such as 2,191,555; 2,163,207; 2,371,928; 2,228,556; 3,591,394 and British No. 1,541,129, describe various ways of generating gas within a cement slurry in order to form a foam cement suitable as a lightweight porous cement, a permeable cement, a pre-stressed cement, a non-shrinking cement and the like. Paper No. 75-PET-10 by B. H. Aldrich and B. J. Mitchell, "Strength Permeabilities and Porosities of Oil Well Foam Cement" , presented at the Petroleum Mechanical Engineering Conference in Tulsa in 1975 and a Colorado School of Mines thesis T-1604 in 1974 by B. H. Aldrich on "Strength, Permeability and Porosity of Cellular Oil Well Cement" discusses the relationship between permeability and porosity and indicates that there is a jump from negligible permeability to high permeability when the porosity exceeds about 52%.
The object of the invention is to provide a method of cementing a well in a formation that is prone to fracturing, by means of a lightweight foam cement slurry, whereby an optimal strength of the set cement column will be obtained.